The Effects of Structure and Flexibility on the Hydrodynamics of Trapezoidal Bio-mimetic Fins

Speaker :
Miss Lan YAO
Department of Mechanical and Aerospace Engineering, HKUST
Date : 16 Jan 2020 (Thu)
Time : 3:30 pm
Venue : Room 2550 & 2551, HKUST (2/F., Lift #27/28)


Millions of years’ natural selection has endowed aquatic animals with the remarkable swimming abilities such as acceleration, efficient cruising in strong currents, quick turns. These extraordinary features can be well adopted to human designed underwater machines and ship propulsion. A practical importance lies in fin propulsions low noise emission, cavitation resistance, impact resistance and efficiency. Bioinspired fin propulsion for manmade watercraft could be the next evolutionary step in the technology of water propulsion systems.

Gradually flexible models undoubtedly offer a deeper insight into biologically inspired and natural fin propulsion. Previous studies regarding gradually varied flexibility however used simple rectangular fins that might differ from fish tail form fins of gradual stiffness. Fin shape and flexibility plays a crucial role in granting fish species their swimming abilities, and it calls for systematic and in-depth studies. In this work, we studied the effect of the trailing edge using curved trailing edge geometries unlike the straight edged fork shape fins in previous literature. We also study the effect of chordwise gradual stiffness using 3D printing technology to produce the model fins of uniform thickness, thus our results are not affected by a varied cross-sectional thickness profile.

This study uncovers the effect on propulsive performance of two natural features of fins, the trailing edge shape and the gradual flexibility. Using particle image velocimetry flow measurement and force measurement in a water tank, we found that the convex trailing edge shape always outperforms the concave shape in the studied Reynolds number range (5500-14500). Towards the high Reynolds number range of our studies the simplest geometry, the trapezoidal fin prevailed. Regarding the advantages of a gradually flexible profile over a uniformly flexible fin, the results show that under any particular Reynolds number setting the gradually flexible fin delivers better propulsive performance. Compared with the uniform fin, the more gradually flexible fins are better in the lower Reynolds range, while those that are less flexible generated more thrust and are more efficient in the high Reynolds range. Our results show that, given a cost-effective manufacturing process of fin propulsors, natural form with gradual flexibility can provide better performance for future watercraft.

(Supervisor: Prof. Huihe Qiu)